There is known an LED illuminator including an LED drive circuit having an LED string in which a plurality of LEDs is connected in series and configured to improve luminance and to prevent a flicker by increasing/decreasing the number of serial stages of the LED string in accordance with an increase/decrease in the voltage of the full-wave rectified waveform and by lengthening an on-state period. Among such LED drive circuits, there is an LED drive circuit configured to improve a power factor and a distortion factor by increasing/decreasing a current that flows through the LED string in accordance with an increase/decrease in the full-wave rectified waveform.
FIG. 11 is a circuit diagram of a light source circuit 2600 described in Patent Document 1. The light source circuit 2600 includes a bridge rectifier 2605 and an LED string. The LED string includes an LED group 2601, an LED group 2602, and an LED group 2603, in each of which a plurality of LEDs is connected in series. The light source circuit 2600 further includes a bypass circuit 2610 configured to operate so as to decrease an effective forward turn-on voltage. The bypass circuit 2610 includes resistors R2 and R3, an enhancement type field effect transistor Q1, and a bipolar transistor Q2.
With reference to FIG. 12, a relationship between a current and a voltage of the light source circuit 2600 is explained. FIG. 12A is a waveform diagram illustrating a relationship between a full-wave rectified voltage waveform V1 corresponding to one period and a time t in the light source circuit 2600 and FIG. 12B is a waveform diagram illustrating a relationship between a circuit current I and the time t of the light source circuit 2600. The scales of the time axis are the same in FIG. 12A and FIG. 12B.
During a period of time t30 during which the voltage of the full-wave rectified voltage waveform V1, which is an output of the bridge rectifier 2605, is less than a threshold voltage (effective forward turn-on voltage) determined by the LED groups 2601 and 2602 in the light source circuit 2600, the current I does not flow through the LED groups 2601 and 2602. During a period of time t31 during which the voltage of the full-wave rectified voltage waveform V1 is greater than or equal to the threshold voltage determined by the LED groups 2601 and 2602 and less than a threshold voltage of the LED string, the current I flows through the bypass circuit 2610 from the LED groups 2601 and 2602. At this time, the bypass circuit 2610 performs a constant-current operation with a current value I31. During a period of time t32 during which the voltage value of the full-wave rectified voltage waveform V1 is greater than or equal to the threshold voltage of the LED string, a current flows through an LED group 3 from LED groups 1 and 2. At this time, if a current with a predetermined value or more flows into the bypass circuit 2610 from the right terminal of the resistor R1, the field effect transistor Q1 cuts off and all the current I comes to flow through the LED group 2603. In this case, the current that flows through the resistor R2 is ignored. When the voltage of the full-wave rectified voltage waveform V1 decreases, the processes take place in the opposite order.
As described above, the light source circuit 2600 has an LED string in which a plurality of LEDs is connected in series and increases/decreases the current I that flows through the LED string in accordance with an increase/decrease in the full-wave rectified voltage waveform V1 as well as increasing/decreasing the number of serial stages of the LED string in accordance with an increase/decrease in the full-wave rectified voltage waveform V1. As a result of this, an attempt to improve the luminance, the flicker, the power factor, and the distortion factor is made to a certain extent.